This study will compare the effects of a 12-week progressive exercise training program on 1) Fibromyalgia (FM) pain and PTSD symptoms, 2) pain threshold and tolerance, and 3) relevant biomarkers and neuromodulators including: a) anti-stress, anti-nociceptive, immune modulating factors such as neuropeptide Y (NPY) and GABAergic neuroactive steroids such as allopregnanolone and pregnanolone (together termed ALLO) b) factors that upregulate the expression of NPY and the GABAergic neuroactive steroids, and otherwise modulate inflammation, such as cortisol, c) excitatory factors such as substance P that directly promote pain transduction and d) pro-inflammatory cytokines such as IL-6 and IL-8 that not only increase pain and inflammation, but also contribute to psychological dysfunction (e.g. anhedonia and depression) via impact on the CNS reward system. This study will focus on Veterans with FM/PTSD. The study design includes a baseline, acute, cardiopulmonary exercise assessment (CPX) that will inform the exercise prescription for a 12- week ?progressive exercise? training program, comprised of three 30-45 minute exercise sessions per week (walking or running, depending on the ability/capacity of the participant). Exercise sessions will be initially supervised by an exercise physiologist in the Clinical Studies Unit (CSU) at the VA Boston Healthcare System and then each participant will transition into the home. Weekly telephone calls by the PI will provide additional motivational support and problem solving. Implementation of the prescribed exercise regimen will also be supported by the use of heart rate and actigraph monitors programmed for the participant to achieve their prescribed heart rate range (HRR). Finally, an ?endpoint? CPX assessment will occur at week 13 to track changes in psychological and neurobiological factors and to delineate their impact on pain and PTSD symptoms. Both CPX, maximum load, exercise tests will be performed in accordance with guidelines published by the American College of Cardiology. Among Veterans with FM/PTSD, changes in the biomarkers assessed after acute, CPX exercise testing will be associated with improvements in pain and PTSD symptoms. Once identified, such biomarkers could be augmented by modification of the exercise regimen to help enhance the anti-stress hormone levels for the FM/PTSD population and experience clinically significant reductions in their symptoms. To obtain sufficient power, 36 participants (18/year) will be recruited. Data from this pilot work will be used to demonstrate feasibility and inform the further development of individually prescribed exercise regimens and a motivationally based exercise behavior change intervention aimed at reducing chronic musculoskeletal pain, including FM, and PTSD symptoms in Veterans. In the short-term, this SPiRE proposal will allow the PI to develop a more effective, motivationally based, exercise behavior change protocol that fosters long-term exercise adherence in patients with FM/PTSD. In the long-term, this intervention will be used as an adjunct to cognitive interventions for these disorders to be further developed and studied via a larger VA, NIH, or DOD-funded grant for which the PI will apply year 2 of the current SPiRE proposal.